Water-based ink for ink jet recording is used generally in the form of a water-soluble dye solution or a liquid obtained by dispersing a water-insoluble color material into water or an aqueous solution including a water-soluble organic solvent. When water resistance is required, hydrophobic fibers are dyed by ink jet textile printing, or hydrophobic fibers or resin film products are dyed by sublimation transfer after ink-jet printing on media such as papers, ink for ink jet recording that includes a water-insoluble color dye is used.
Characteristics required for the ink for ink jet recording that includes a water-insoluble color material are as follows: (1) ejection stability while preventing nozzle clogging; (2) storage stability; (3) high-quality recording images resulting from uniformity in the direction of ink ejected, the ejection amount, and the dot shape; (4) quick drying and fixation; and (5) high print density.
In an ink jet recording system, ink should be ejected from thin nozzles as ink droplets. Therefore, (1) is particularly important. When the ink jet recording system is of the drop-on-demand type, the ejection of ink from the nozzles is stopped temporarily even during continuous operation. This may lead to an ejection failure in long continuous operation.
Unlike a water-soluble dye, the water-insoluble color material tends to cause clogging, e.g., because precipitates (aggregates) are generated by the degradation of dispersion stability and the solvent evaporates from the nozzles. In particular, when the apparatus is not operated for a long-time, clogging is likely to occur due to evaporation of the solvent.
To suppress evaporation of the solvent, a low-volatile solvent or humectant is added. Alternatively, a hydrotropic agent is added so that even if a portion of the solvent included in the ink evaporates, the ink can be redispersed when it comes into contact with ink supplied further from an ink chamber. Examples of the humectant include glycols such as glycerin and polyethylene glycol. Examples of the hydrotropic agent include urea and urea compounds.
However, when used in a small amount, these additives are not effective, and when used in a large amount, the ink viscosity is raised or the surface tension is significantly changed, moreover, the drying property of printed materials is degraded.
To solve the above problems, e.g., JP 8-41396 A discloses that a solvent including water, glycerin, and propylene glycol is used to prevent nozzle clogging and to achieve quick-drying of the ink on a recording media after printing. In this document, when recording can be performed even after the nozzles are left for 120 seconds or more, the ink is regarded as being “excellent”. However, the document is not sufficient to solve the issues of redispersion property and ejection performance of the ink after stopping the operation for a long time, e.g., after the nozzles are left overnight.
JP 10-251579 A discloses a technique to prevent clogging by using an ethylene glycol lower fatty acid ester compound that can improve the solubility of a dye in the ink and serve as a humectant. Although this technique can prevent clogging when a water-soluble dye is used as a colorant, it is not effective when a water-insoluble dye is used as a colorant.
JP 2000-265098 A discloses a technique to prevent the evaporation of a solvent by forming a liquid crystal structure at the gas-liquid interface with the addition of a polyoxyethylene alkyl ether compound that includes an alkyl group having a carbon number of 14 to 23. However, as is evident from a paragraph [0016] of the document, which states that “a water-soluble dye is preferred”, the dispersion stability and redispersion property of the color material can be a problem when this technique is applied to a water-insoluble color material.
As an example of the use of a water-insoluble color material, JP 10-95944 A discloses a technique for producing an ink that includes a pigment, a humectant, and a long chain polyethylene oxide-containing dispersant. However, this technique also cannot prevent the precipitation of a solid matter made of the pigment and the dispersant. Therefore, excessive cleaning operation is necessary to wash off the solid matter.
To solve the above problems of a conventional ink for ink jet recording, an object of the present invention is to provide an ink for ink jet recording that not only can maintain required performances such as storage stability and high-quality recording images, but also can effectively prevent nozzle clogging, exhibit good dispersion stability, and achieve a high redispersion property that ensures stable ejection both during long continuous operation and after stopping the operation of an apparatus for a long time, even if a water-insoluble color material is used as a color component.
The present inventors have considered that when an ink composition has a high redispersion property such that it can be redispersed easily without any aggregation by the addition of a small amount of water even after drying, the ink composition can be reliable enough to prevent nozzle clogging of an ink jet head. The study conducted by the present inventors showed that the use of a specific compound can provide an ink with the above characteristics.